I've been a fan of Edgar Rice Burroughs's Barsoom stories for, oh, gods, thirty-two years. Approximately as long as I've been playing D&D, now that I think of it. For most of that time, I've wanted a Barsoomian campaign. Now that there's finally going to be a John Carter movie (yay!) opening just days after my next birthday (double yay!), I've been wanting a Barsoomian campaign even more. So for months I looked at the various game systems I've got, trying to work out what rules to use; do I go with Space: 1889? D20 Modern/Past/Future? GURPS (which, while I'll play, I swore I'd NEVER try to run)? I thought I'd never work out what I wanted. Finally, I think I've got it.
I had two sources motivating me; the first six Barsoom books (Princess, Gods, Warlord, Thuvia, Chessmen, and Mastermind), and the Larklight trilogy by Philip Reeve. So, Planetary Romance meets Steampunk Boys' Adventure; just the sort of thing to make my heart go pitter-pat. And keep my players interested, since however close I come to Burroughs's Barsoom, it'll still be different enough to keep my players guessing. Also, by stealing from multiple sources, I manage to come up with something that feels original, while still evoking a setting and style that's 90+ years old.
For setting and background I'm using a 60/40 combination of Adamant's MARS and Space: 1889, but with all the game mechanics removed. I wanted to use 4E rules, so for the game mechanics, it's almost entirely Gamma World, with some D&D thrown in to fill in the gaps. Monsters and equipment are drawn from Gamma World and the 4E Dark Sun with only cosmetic changes here and there. Omega Tech will be used as is, although without the computers. Terrestrial firearms will probably use the crossbow stats, but I haven't decided yet.
Character creation gave me the hardest time, but I think I've finally got that licked. Those playing Earth men (and women) will use the Martial classes from D&D, or the Artificer with all the Arcane flavor replaced with SCIENCE. What's a Steampunk adventure without the occasional Mad Scientist? When their characters arrive on Mars, they all take a +4 to Strength and -2 to Dexterity (for the low gravity), and a -1 to Constitution (for the thin air). Maybe only a -1 to Dex; I haven't made up my mind yet.
Those playing Martians will use the Gamma World rules for character creation. Mostly. Red Martians, as psychic athletes, all use the Engineered Human for their primary origin and their choice of Empath, Exploding, Hypercognitive, Mind Breaker, Mind Coercer, Prescient, Telekinetic, or Vampiric for their secondary origin. At each level where they gain an Alpha Mutation, they select a Psi power as a permanent power.
Green Martians, being monstrous barbarians, all use the Giant for their primary origin and their choice of Felinoid, Hawkoid, Plaguebearer, Regenerator, or Saurian as their secondary origin. For their Alpha Mutations, they choose Bio powers. And for all of these, the names and flavor text will be adjusted. The premise of this universe IS silly, but not quite as silly as the standard Gamma World campaign. Also, given that the Green Martians are psychic too, I might consent to a player choosing one of the Red Martians' options for their secondary origin, but probably only if there are multiple Green characters in the party.
Grey Martians are H.G. Wells's WotW nightmares, and they use the Octopoid for their primary origin and their choice of Antimatter Blaster, Doppelganger, Ectoplasmic, Electrokinetic, Entropic, Gravity Controller, Magnetic, Photonic, Radioactive, or Temporal for their secondary origin. For their Alpha Mutations, they choose Dark powers. It's all that mucking about with SCIENCE that warps them so.
White Apes are, well, white apes (Martian style), and use, well, Simian for their primary origin and their choice of Arachnoid, Cockroach, or Yeti (all with MASSIVE descriptive changes) for their secondary origin. For their Alpha Mutations, they choose Bio powers.
Finally, Synthe-Men are artificial people created to build the original canals, and to autonomously maintain them, as well as the atmosphere factories. Player characters would be those rare individuals who develope personalities and drives outside their programmed functions. They all use Fungoid as their primary origin and their choice of Android, Gelatinous, Plant, Plastic, Rat Swarm, or Shapeshifter for their secondary origin. For their Alpha Mutations, they choose Bio powers, although I'm considering letting them use some of the Dark powers.
So, that's what I've got so far. Any advice on the above undecided points is most welcome. Also, if you think of something I've overlooked, please let me know. So, questions and/or comments?